1. Field of the Invention
The invention relates to a method for producing samples for transmission electron microscopy, and to an apparatus suitable for carrying out the method.
2. Description of the Related Prior Art
Since their introduction in the 1930s, transmission electron microscopes have found wide application in various branches of science and industry. Because of the resolution which is substantially better by comparison with light microscopy and lies in the sub-angstrom range for the currently best appliances, the microstructure and nanostructure of various types of preparations can be examined in great detail.
Particularly in the field of characterization of solids, information can be obtained on the atomic scale with the aid of transmission electron microscopy (TEM). In this case, it is possible on the one hand to examine the arrangement of the atoms, while on the other hand it is also possible to access the chemical composition and the electronic structure with the aid of dedicated analysis methods such as x-ray spectroscopy and electron energy loss spectroscopy. The presence of fewer, in exceptional cases even individual, atoms can be demonstrated in the case of electron energy loss spectroscopy.
Up to a few years ago, the electron optical systems used in transmission electron microscopes were not sufficiently good to allow images and analyses at the theoretical spatial resolution limit. The introduction of highly complex aberration correctors and monochromators, which enable control via limiting parameters such as the aperture aberrations and chromatic aberrations of electron lenses, a very much closer approach with regard to spatial resolution limit is presently being made. Whereas it was possible in the 1990s to attain spatial resolutions in the sub-angstrom range only by means of very high acceleration voltages of 1 MeV and more, and to attain the very short electron wavelengths associated therewith, it is presently possible through the use of corrector lenses to advance into the sub-angstrom resolution range even with moderate acceleration voltages of, for example, 80 to 300 keV. This results in advantages with respect, inter alia, to the frequently problematic beam damage of the very thin preparations, which should have only a very slight thickness (order of magnitude of a few 10 nm) for highly resolved imaging and analysis in the examined range.
Consequently, the question of efficient and low-damage methods for preparing the samples for transmission electron microscopy, which are also referred to below for short as TEM samples, arises to an increasing extent. The invention relates in this case specifically to a method for producing TEM samples in which is prepared from a substrate of a sample material a sample which has a wedge-shaped sample section which is bounded by wedge surfaces and has in the region of the wedge tip at least one electron-transparent region.
It is true that thinning is possible in principle in a purely mechanical way, but it requires great manual skill in order to lead to reproducible sample quality at least to some extent. Moreover, there is presently a multiplicity of partially very complex technologies for producing adequately thin, electron-transparent areas on TEM samples. These include, in particular, mechanical prethinning (grinding, polishing, cavity grinding), which is followed by an ion beam etching process, the cutting out of thin sections with the aid of a focused ion beam, and ultramicrotomy.
DE 10 2004 001 173 B4 describes a method for preparing TEM samples in the case of which material is removed from a substrate of a sample material by means of ultrashort pulse laser ablation in a vacuum chamber, this being done in such a way that there remains a narrow web which is subsequently bombarded in a flat angle with an inert gas ions such that an electron-transparent area is produced in the region of the web. Numerous further conventional preparation methods for TEM samples are described in the introduction to the description in this publication.
The methods used for ion beam thinning go hand in hand in principle with the formation of a near-surface amorphization/damage of the sample material, the extent of which is a function of the acceleration voltage of the ions. The amorphization/damage can be more than 10 nm, for example, when use is made of 3 keV gallium ions, while damage thickness of between 3 and 6 nm is frequently observed in the case of typical low-angle ion etching at 3 keV. According to results of relevant investigations, a reduction to below 1 nm is scarcely possible even when use is made of low-energy ion beams (typically 200 eV to 500 eV energy).
For the special case of substrates made from materials with defined cleavage planes, the so-called “Small Angle Cleavage Technique (SACT)” has been proposed (see, for example, J. P. McCaffrey, Ultramicroscopy 38, 149 (1991) or J. P. McCaffrey, Microscopy Research and Technique 24, 180 (1993)). In the case of a piece of a sample material thinned by grinding, with this technique a diamond tip introduces parallel scratch paths of approximately 500 μm width in the direction which assumes an angle of approximately 18.5° relative to a known cleavage plane of the sample material. Subsequently, the preparation is broken along the scratch paths and along the material-specific cleavage plane. In favourable cases, the breaking of the samples results in a tapering preparation with an enclosed angle of approximately 18.5° between the fracture surfaces. In the ideal case, electron-transparent areas which can be examined by means of TEM are located at the outermost end of the wedge-shaped sample section. An advantage of this technique consists in that because of the missing action of ion beams, the prepared TEM samples have no amorphization caused by preparation, and are free from chemical contaminations. Ideally, atomically flat surfaces result on the wedge-shaped sample section. However, the possibilities for using the method are restricted, because it can be used in essence only with sample materials with crystallographically defined cleavage planes. In addition, the method does not lead as a rule to reproducible results. Furthermore, it is difficult to impossible for it to be automated. Finally, it is scarcely possible to implement even a target preparation, since the exact location of the crack initiation cannot be determined accurately enough owing to the coarse scoring.
It is particularly difficult and complicated to prepare cross-sectional preparations that are intended to enable examination of layers, layer systems or structured surfaces substantially perpendicular to the surface normals of the layer structures. Such questions arise to a particular degree in the semiconductor industry, where many thousand cross-sectional preparations are inspected per year in transmission electron microscopes for the purpose of quality control and fault analysis. The benefit of transmission electron microscopy resides here, inter alia, in an extremely high spatial resolution which enables the examination of ever smaller structures on structured semiconductor components. It is becoming increasingly evident that the preparation of high-quality TEM samples with sufficiently thin electron-transparent areas is the step which is limiting throughput. This circumstance is frequently countered by procuring a number of preparation machines, but this, by way of example, is associated with high costs in the case of systems for focused ion beam (FIB) processing.
A plurality of boundary conditions which are to be rendered compatible only with difficulty are typically to be considered when examining structured semiconductor components in the course of quality control and fault analysis. What is frequently involved is to obtain cross-sectional preparations from structured sample material in the case of which the electron-transparent area suitable for examination lies at a position which can be determined with great accuracy and can ideally be derived from the positioning in machines from the process chain. This task is denoted in specialist circles as “target preparation” (or “site-specific preparation”). This target preparation should be performed as quickly as possible so that the results of the quality control can be fed back quickly into the production process of the components. Furthermore, the sample preparation technology should be optimized so as largely to avoid introducing artefacts into the preparation. Typical artefacts include near-surface amorphization (problematic, for example, when cutting out with the aid of a focused ion beam) as well as structural modifications, diffusion, phase changes owing to heat input etc. Finally, the attempt is made, particularly in industrial use, to automate the preparation process as far as possible in order to be able to attain reproducible preparation results in as wide a process window as possible.